Color-changing nanoparticles had always been possible in liquids. Now, a team of chemist at the University of California have figured out how to replicate the plasmonic color-switchable films of silver nanoparticles (AgNPs) in solids too.
Such color-switching capability has numerous applications. For example, it could inspire new types of color display signage, information encryption methods, and sensors too.
In a statement on the project, professor of chemistry at the university, who also leads the study Yadong Yin said:
“Rapid and reversible tuning of plasmonic color in solid films, a challenge until now, holds great promise for a number of applications. Our new work brings plasmonic metal nanoparticles to the forefront of color-converting applications.”
Plasmonic metal nanoparticles – like gold and silver – have an unusual optical property. They can absorb and scatter light at a specific wavelength effectively.
In addition, you can alter the color of these metal by changing the distance between their particles. Inspired by this unique feature, Yin and his team created their plasmonic color-switching film.
How the Researchers Fabricated Color-Changing Nanoparticles
Yin and his team coated a glass substrate with a layer of borax. Then they created a film by spraying AgNPs over the borax.
According to the researchers, there are capping ligands on the surface of each AgNP. This provides a form of distance between the AgNPs, preventing them from clumping together.
That’s where the borax comes in.
Borax turns to boric acid in the presence of water and releases hydroxyl ions. These ions then remove protons from a group of ligands and add a negative charge to the AgNPs.
As you already know, negative charge repels. So, the repulsion force pushes the nanoparticles apart, giving them a new inter-particle distance.
As a result, the nanoparticles’ color changes from pink to reflect yellow. Conversely, removing the moisture converts the boric acid back to borax, and triggers a color change back to pink.
Yin noted:
“Through this mechanism, we could rapidly achieve plasmonic color switching of the AgNP film in the presence or absence of moisture. In our experiments, we exposed the AgNP film to the moisture of 80 percent relative humidity and found the film changed colors from pink to red, orange, and finally yellow.”
What are the possible applications of the color-changing nanoparticles?
According to the researcher, it could provide a rapid and touchless way of encrypting information. Manufacturers can also embed the color-changing nanoparticle system in their product as a form of product authentication.
Other potential applications include real-time calorimetric health or environment monitoring and secure communication.
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